1. Field of the Invention
The present invention relates to a gear ratio variable type steering device, and in particular to a gear ratio variable type steering device which is capable of varying a linear displacement of a rack bar based on the revolution of a steering wheel in accordance with the speed of a vehicle.
2. Description of the Background Art
Generally, a steering device of a vehicle is constructed in such a manner that a linear displacement of a rack bar is constantly generated when a driver rotates a steering wheel. The thusly generated linear displacement is expressed as 40 mm/rev. The gear ratio affects the characteristics of the steering operation of the vehicle.
FIG. 1 is a perspective view illustrating a conventional steering device. As shown therein, the conventional steering device includes a steering wheel 10 for generating a steering force based on a driver""s operation, a column 20 connected with the lower potion of the same, a universal joint 30 connected to the lower portion of the column 20, and a gear box 40 installed below the same.
FIG. 2 is a front view illustrating the gear box, and FIG. 3 is a cross-sectional view of the same. As shown therein, a housing 41 which forms the outer construction of the gear box 40 is provided.
In addition, a hollow input shaft 42 is protrudely installed in the housing 41 for receiving a steering force generated by the steering force.
A pinion gear 43 is integrally provided at the lower portion of the input shaft 42. A rack bar 44 having a rack gear 45 at one side of the same is installed at the lower portion of the housing 41 for a linear movement based on an engagement with the pinion gear 43.
At this time, the rack bar 44 is displaced axially across the rack gear 45, and a ball joint 50 connected with the steering member 60 is installed at both ends of the rack bar 44.
In the thusly constructed conventional steering device, when a driver rotates the steering wheel 10, the input shaft 42 is rotated, and the rack bar 44 is linearly operated based on the gear ratio which is determined when designing the vehicle and angular-operates the steering member 60.
In the conventional steering device, since a certain gear ratio is constantly generated irrespective of a low speed or high speed operation, the distance of the movement of the rack bar becomes same based on the revolution of the input shaft.
Therefore, it is impossible to generate a large movement of the rack bar by rotating the steering wheel by a small amount at a low speed operation and generate a small movement of the rack bar by rotating the steering by a large amount at a high speed operation.
Namely, the conventional steering device is designed based on a constant gear ratio. Therefore, it is needed to rotate the steering wheel by a large amount in the case of the low speed operation and parking operation which need a large steering angle for thereby causing much inconvenience. In the case of the high speed operation which needs a small steering angle, the large steering angle is generated which is not proper to the high speed operation, so that it is impossible to obtain a certain steering stability.
Accordingly, it is an object of the present invention to provide a gear ratio variable type steering device which overcomes the problems encountered in the conventional art.
It is another object of the present invention to provide a gear ratio variable type steering device which includes a variable gear mechanism between a steering wheel and a gear box for varying a gear ratio based on the speed of a vehicle.
To achieve the above objects, there is provided a gear ratio variable steering device a gear ratio variable type steering device which comprises a variable gear mechanism which includes an input shaft for receiving a rotational force of the steering wheel, an output shaft for transferring a rotational force to the gear box, a variable pulley installed in the outer portions of the input shaft and the output shaft and moved in the axial direction, a belt for connecting the variable pulley, and a motor for varying a gear ratio by moving the variable pulley in the axial direction.